A number of medical ailments are treated and/or diagnosed through the application of a magnetic field to an afflicted portion of a patient's body. Neurons and muscle cells are a form of biological circuitry that carry electrical signals and respond to electromagnetic stimuli. When an ordinary conductive wire loop is passed through a magnetic field or is in the presence of a changing magnetic field, an electric current is induced in the wire.
The same principle holds true for conductive biological tissue. When a changing magnetic field is applied to a portion of the body, neurons may be depolarized and stimulated. Muscles associated with the stimulated neurons can contract as though the neurons were firing by normal causes.
A nerve cell or neuron can be stimulated in a number of ways, including indirectly via transcranial magnetic stimulation (TMS), for example. TMS uses a rapidly changing magnetic field to induce a current in a nerve cell, without having to cut or penetrate the skin. The nerve is said to “fire” when a membrane potential within the nerve rises with respect to its normal negative ambient level of approximately −90 mV, depending on the type of nerve and local ionic conditions of the surrounding tissue.
The use of magnetic stimulation is very effective in rehabilitating injured or paralyzed muscle groups and may prove useful in other therapies involving peripheral nerve stimulation including, but not limited to, pain mitigation, stimulation of neovascularization, wound healing and bone growth.
Magnetic stimulation also has proven effective in stimulating regions of the brain, which is composed predominantly of neurological tissue. One area of particular interest is the treatment of depression. It is believed that more than 28 million people in the United States alone suffer from some type of neuropsychiatric disorder. These include conditions such as depression, schizophrenia, mania, obsessive-compulsive disorder, panic disorders, and others. Depression is the “common cold” of psychiatric disorders, believed to affect 19 million people in the United States and possibly 340 million people worldwide.
Modern medicine offers depression patients a number of treatment options, including several classes of anti-depressant medications (e.g., SSRI's, MAOI's and tricyclics), lithium, and electroconvulsive therapy (ECT). Yet many patients remain without satisfactory relief from the symptoms of depression. To date, ECT remains an effective therapy for resistant depression; however, many patients will not undergo the procedure because of its severe side effects.
Recently, repetitive transcranial magnetic stimulation (rTMS) has been shown to have significant anti-depressant effects for patients that do not respond to the traditional methods. The principle behind rTMS is to apply a subconvulsive stimulation to the prefrontal cortex in a repetitive manner, causing a depolarization of cortical neuron membranes. The membranes are depolarized by the induction of small electric fields in excess of 1 V/cm that are the result of a rapidly changing magnetic field applied non-invasively.
To generate a magnetic pulse that is capable of providing a therapeutic effect on a patient, TMS, rTMS and Magnetic Seizure Therapy (MST) treatments all require a great deal of electrical power, typically in the range of several hundred joules (J) per pulse. Various attempts to optimize the design of the coil used in such treatments have not been able to substantially mitigate the need for a great deal of electrical power. For example, to cause a stimulation coil to generate trains of rapid rTMS pulses, thousands of watts (W) of power are typically delivered to the coil. This amount of power leads to rapid coil heating. The amount of coil heating is so great that the coil often is heated to the point at which it would be uncomfortable or unsafe to use the coil on a patient. Thus, attempts have been made to cool stimulation coils using water, air or oil. Unfortunately, these cooling mechanisms are cumbersome, add complexity to the magnetic stimulation system, are expensive and are sometimes adversely affect the performance of the stimulator. A more advantageous approach would be to reduce the amount of power required by the magnetic stimulation device to generate a therapeutically-equivalent magnetic pulse.